The present invention relates broadly to constant false alarm rate detectors and in particular to a doubly adaptive CFAR apparatus.
In the prior art, spatially-varying clutter statistics and dense maritime traffic environments impose difficult requirements on a conventional adaptive detector. This is because the detector fails to maintain a constant false alarm rate as clutter statistics vary and also fails to accommodate target-to-target interference as target enters the CFAR block. There is a further problem for the conventional adaptive detector which fails to accommodate the target-to-target interference as targets pass through the clutter cells in the detector. The clutter cells are usually referred to as a CFAR block which forms the clutter average. When the target enters the CFAR block, the target signal will raise the clutter average and degrade the ability to detect the nearby target incidently located in the detection cell. The detection degradation due to the target interference has been found to be unacceptable especially if radar operates in very heavy target environments. For example, a target with signal to noise ratio of 20 dB will yield 1.5 dB detection degradation if target occupies only 2 percent of a CFAR block. The degradation is found to be even more severe if target range extent or amplitude is large. Multiple targets such as a fleet of naval vessels or aircraft are not unusual in the real sea environment. Previous attempts to alleviate the target interference include the technique of increasing the CFAR block size. However, the technique can only alleviate but not totally eliminate the interference effect. Furthermore, there are some cases for example, such as high PRF radar, where the CFAR block cannot be extended. To eliminate the above-mentioned problems for a CFAR detector, a unique doubly adaptive detector apparatus is presented.